(1) Field of the Invention
The present invention relates to an ion implanter for implanting ions in a semiconductor wafer, and anion implantation control method thereof. In particular, the present invention relates to an ion implanter capable of improving dose uniformity and suppressing generation of contaminated particles on a wafer, and an ion implantation control method thereof.
(2) Description of the Related Art
In an ion implanter, a technique for improving dose uniformity and a technique for reducing the number of particles are fundamental techniques, and various modifications have been made heretofore.
First, description will be given of the technique for improving dose uniformity.
An ion implanter mainly includes an ion source for generating ions, an extraction electrode (an accelerating section), a mass analyzing section for acquiring desired ions, an accelerating/decelerating section for acquiring desired energy, and an ion implanting section for holding a wafer to implant ions in the wafer.
A wafer holding method of the ion implanting section includes batch processing in which plural wafers are held in a disc, and single wafer processing in which a single wafer is held.
In the former batch processing, an ion beam is made stationary and a mechanical scan is performed in a radius direction while a disc is rotated at a high speed, so that dose uniformity is secured. This type is generally adopted in a high current ion implanter.
In the latter single wafer processing, there is mainly adopted a hybrid scan in which an ion beam scan section mechanically scans a wafer with an ion beam in one direction. This type is generally adopted in a medium current ion implanter. In recent years, a high current ion implanter of the single wafer processing type is also developed. Thus, there are devised an ion implanter of a hybrid scan type similar to that of the medium current ion implanter, an ion implanter for making an ion beam stationary without scan to mechanically scan a wafer in a horizontal direction and a longitudinal direction, an ion implanter for generating a ribbon beam obtained by expanding an ion beam in a horizontal direction, to mechanically scan a wafer in a longitudinal direction, and the like. In any of the aforementioned ion implanters, a mechanical scan is performed in at least one direction. In order to render an amount of ions implanted in a wafer uniform, an ion beam current is measured, and a speed of a mechanical scan mechanism is changed in accordance with variation of an amount of the measured ion beam current. More specifically, a control system used in each of the aforementioned ion implanters controls a mechanical scan speed in such a manner that a dose at the time when a wafer is scanned once with an ion beam (at the time when a disc rotates once in a batch processing ion implanter) is invariant. However, in some cases, an amount of ions implanted in a wafer becomes nonuniform in a mechanical scan direction due to a characteristic of an ion implanter, and other reasons.
In order to solve the aforementioned problem, for example, JP2001-167727A discloses an ion implantation system. This ion implantation system feeds back characteristic data of a wafer measured after being subjected to ion implantation treatment, corrects a mechanical scan speed, and prevents nonuniformity of ion implantation due to a characteristic of an ion implanter.
Next, description will be given of the technique for reducing the number of particles.
Materials for the ion source, the extraction electrode, a wafer holding section of the ion implanting section, and the like are devised to suppress generation of particles. Further, a particle generation source is identified and is subjected to maintenance such that particles on a wafer are reduced in number. For example, JP2004-356297A discloses a particle reduction method. Herein, plural particle monitors are placed over a whole beam line including an ion source, a mass analyzing section, an accelerating/decelerating section and the like in addition to an ion implanting section in an ion implanter, so that a site where particles are generated can be identified immediately. Further, if the site where particles are generated is located on the beam line, a diameter of an ion beam is set to a predetermined diameter by a rise in extraction/acceleration voltage, and the like, so that generation of the particles can be suppressed.
In the ion implanter and the ion implantation method that are well-known, however, the following problems arise.
The ion implantation system disclosed in JP2001-167727A can feed back and correct the characteristic data of the ion implanter. However, if the ion implanter is operated for a long period of time, an ion beam current suddenly varies due to a change in state of the ion implanter during the ion implantation treatment in many cases. If the sudden variation of the ion beam current occurs, the characteristic data of the ion implanter cannot be corrected. In addition, if the ion beam current suddenly varies upon implantation of ions in a wafer having a characteristic to be measured, the characteristic data of the ion implanter is corrected on the basis of the characteristic of the wafer; therefore, there is a possibility that dose uniformity becomes poor. In order to address the sudden variation of the ion beam current, it is considered that an interlock is provided for suspending processing if the ion beam current varies significantly beyond a set value. However, if a range of the set value is too narrow, there is a fear that a wafer processability deteriorates. In contrast, if such a range is too wide, dose uniformity becomes poor, leading to the same adverse result as the aforementioned description.
On the other hand, with the particle reduction method disclosed in JP2004-356297A, an extraction/acceleration voltage is allowed to rise upon adjustment of the diameter of the ion beam, and an acceleration voltage at a later stage is allowed to drop. In many cases, however, the extraction/acceleration voltage is already maximum in a normal state for the purpose of acquiring a larger amount of ion beam currents. This method cannot be applied in such a case. In addition, if the acceleration voltage is allowed to drop at a later stage like a high current ion implanter, there is a possibility that an increase in energy contamination is caused. Further, in addition to the diameter of the ion beam, a center position of the ion beam is also deeply involved in generation of particles. Upon maintenance of the ion implanter, an orbit of the ion beam disadvantageously deviates due to changes in state of the ion implanter, such as erroneous attachment of an exchanged ion source, a chipped slit in an extraction electrode, and adhesion of deposits, so that an area, irradiated with the ion beam, of a component located on the orbit of the ion beam is enlarged. As a result, particles are easily generated, leading to deterioration in yield of a semiconductor device product and reduction in operating time of the ion implanter.